The present invention pertains to vertical cavity surface emitting lasers (VCSELs), and particularly to VCSELs having current confinement. More particularly, the invention pertains to VCSELs having refined current confinement caused by an implant or diffusion not having unwanted damage in the VCSEL structure.
Several patents address the issue of current confinement. U.S. Pat. No. 5,115,442 reveals a structure having a semiconductor quarterwave stack in both mirrors. The entire semiconductor epitaxial structure is deposited first, followed by a deep proton implant to confine the current. This is a commonly used structure. Its drawbacks include the fact that the top mirror is several microns thick, and therefore the implant must be so deep that one is limited in how small the current path can be made. Since the depth is so large, and there is significant straggle of implanted ions, the diameter of the current confined region cannot be made as small as one would like. This makes it more difficult to produce a single mode device and more difficult to keep the current required to reach the threshold for lasing small. In addition, damage is produced in proximity to the active region by the implant, which could eventually limit the lifetime of the device. The limit on size restricts performance. Furthermore, there are reliability concerns due to the proximity of the implanted region next to the gain region.
A second related U.S. Pat. No. 5,256,596 also provides for current confinement using ion implantation, but has a mesa etched before the implanting, so the implant depth is smaller. In that structure, a buried implant is used to provide current confinement. However, the entire epitaxial structure is deposited first, and a mesa must be etched before ion implant, in order to place the implant at the right depth, since the range of dopant atoms is quite small compared to protons. In fact, one can wonder whether the structure shown in FIG. 3 of that patent is even feasible, since it would require the implant of p- type atoms several microns below the surface. The disadvantages of this approach are that it results in a non-planar surface, and requires implantation through or close to the active region, thereby resulting in potential reliability problems.
U.S. Pat. 5,475,701, by Mary Hibbs-Brenner and issued Dec. 12, 1995, is hereby incorporated in this specification by reference.